There is currently an increasing demand for energy sources such as petroleum and natural gas. Due to this increased demand, there is increasing installation and use of line pipe in severe environments such as highly corrosive environments, deep oceans, and permafrost. Particularly in corrosive environments, there is a tendency for hydrogen-induced cracking (also referred to below as HIC) to easily develop.
Accordingly, there is a demand for line pipe which is manufactured from a steel which has corrosion resistance and which in particular does not readily undergo HIC (referred to below as HIC-resistant steel). The mechanism of HIC and the properties required of HIC-resistant steel will be described below.
(1) Mechanism of HIC
When a pipe is used in environments containing hydrogen sulfide (H2S), hydrogen ionizes and is occluded into the pipe. The occluded hydrogen is trapped by inclusions in the pipe, and the trapped hydrogen develops a high stress inside the pipe and causes cracking inside the pipe.
(2) Properties Required of HIC-Resistant Steel
In order to suppress the occurrence of HIC, it is preferable to decrease the amount of inclusions which trap hydrogen occluded in a pipe. For this purpose, it is necessary to maintain a high degree of cleanliness of steel. In addition, a low temperature transformation structure (martensite, bainite, or the like) easily forms in the location of center segregation, and HIC easily develops in this low temperature transformation structure. Therefore, it is necessary to reduce the contents of elements such as C, Mn, and P, thereby suppressing the occurrence of segregation.
In the manufacture of HIC-resistant steel, in order to obtain a steel having the desired properties set forth above in (2), the steel basically has decreased contents of C and Mn, and it becomes necessary to supplement the strength by the addition of other alloying elements. In order to guarantee strength, the steel generally contains Nb and high temperature heating is performed on the steel so that solution strengthening of Nb can be utilized. If low temperature heating is carried out on the Nb-containing steel, Nb carbonitrides, which are one type of inclusions which cause HIC, are formed. Therefore, in the case of an Nb-containing steel, it is essential to carry out high temperature heating in order to guarantee HIC resistance. In addition, in the hot rolling stage, high temperature finish rolling at or above the transformation point is employed so as to obtain a uniform structure.
The following are examples of measures which have been disclosed for increasing the quality of HIC-resistant steels.
Patent Document 1 discloses that the presence of MnS in steel causes cracking to occur with MnS serving as a starting point, and the susceptibility to cracking increases by extension of MnS to an elongated form at the time of rolling. Therefore, by decreasing the S content in steel and adding Ca and REM to the steel, S in the steel is transfoimed into fine spheroidized CaS and REM sulfides.
Patent Document 2 discloses that in the location corresponding to the center segregation region of a cast slab, hard structures such as martensite and bainite are formed by segregation of C, Mn, P, and the like, and these hard structures become a path for transmission of cracks. Therefore, the formation of a hard structure is prevented by decreasing the concentration of C, Mn, P, and the like in steel and carrying out soaking in order to decrease segregation by diffusion.
Patent Document 3 discloses preventing center segregation itself by performing bulging of a cast slab at a stage during continuous casting in which unsolidified molten steel remains followed by rolling reduction.
Patent Documents 4-6 disclose that as the strength specifications demanded of recent steels has increased, just the above-described single measure against the occurrence of center segregation or the formation of MnS is inadequate. Therefore, Cu or Ni is added to steel in order to form a protective film on the surface of the steel, thereby suppressing the infiltration of hydrogen into the steel, and this measure is combined with the addition of Cr, Mo, or the like or thermo-mechanical controlled process (TMCP) at the time of rolling.